In many areas of application for microcontroller control systems, where it is important to ensure reliability of control routines being executed, the control system should be able to automatically detect malfunctions or faults in circuits and electrical connections that may occur. For instance, malfunctions or faults in the connections to the printed circuit board or in the bond wire connection to the pads of the integrated circuit should be detected.
In the automotive field for instance, short-circuits and/or disconnections as a result of jolts and other accidental stresses are relatively frequent. One or more input channels of the analog-to-digital converter that are integrated on the microcontroller chip of the control system of the engine and of other functional devices and subsystems of the vehicle may inadvertently be short-circuited or disconnected. This would cause undesired effects and/or spurious reactions of the control system since they would be without the correct information concerning an important functioning parameter.
To make known any failures such as the above mentioned ones, one technique is to (on the printed circuit board or integrated on the same microcontroller chip containing also the analog-to-digital converter) connect resistors from the input node of the converter to a node at a certain reference potential to force a well defined value on the input of the converter. For instance, the reference potential may be the supply voltage and/or to the virtual ground node of the circuit for instance. This allows detection of anomalous situations, such as an open circuit.
However, the introduction of these resistors may determine set time constants having such a value that they will influence a correct functioning of the converter, or at least noticeably lengthen the sampling time and thus the total conversion time. As a result, in the successive approximations analog-to-digital converter, such as those commonly used in these integrated systems, the conversion time may become excessively long and thus noticeably hinder the performance of the control system.